1. Field of the Invention
The present invention relates to a transmitter suitable for transmitting various measured values through a two-wire transmission line.
2. Description of the Prior Art
In order to transmit outputs from a differential pressure transmitter, an electromagnetic flowmeter or the like representing measured values to a remote location according to conventional industrial measurement techniques, a unique signal having a current level within a range of 4-20 mA is used. Thus, an analog signal having a current level selected from this range represents a measured value. Such differential pressure transmitters, electromagnetic flowmeters and the like are normally arranged in a distributed manner to monitor industrial process states in a wide physical area. Maintenance personnel must travel extensively to maintain and inspect the distributed measuring instruments so as to perform adjustments and check the operating conditions thereof. In order to eliminate such time-consuming maintenance or the like, existing equipment is utilized to achieve remote control operation of the measuring instruments, as described in U.S. Pat. No. 4,520,488.
As shown therein, a transmitter is bridged to a two-wire transmission line to transmit a digital signal. After the digital signal is received by a receiver, the transmitter stops transmitting an analog measured value signal and responds to a digital signal from a communicator. The response signal is then received by the communicator. A similar mode of operation is achieved for digital signal communication between the communicator and the transmitter.
Since when the transmitter communicates with the communicator by means of the digital signal, the transmitter stops transmitting the analog signal, i.e., stops transmitting the measured value, if the measured value concurrently changes, the changed measured value cannot be immediately transmitted to the receiver. Therefore, the receiver cannot perform an immediate control operation according to the changed measured value. This impairs the ability of the state of equipment to be controlled to follow changes in the measured values.